Abstract
Flexible direct current (DC) grid can realize large-scale renewable energy, wide-area coordinated complementation, and reliable power transmission. It is an important development that can be used to support high-voltage and large-capacity flexible DC transmission in the future. The short-circuit current of the DC line is one of the important bases for the selection of key main equipment parameters such as converter valves, DC circuit breakers, and reactors in the flexible DC grid. In this paper, a flexible DC grid equivalent circuit network model with alternating current (AC) feed-in is established. Aiming at the monopolar ground fault of the flexible DC grid grounded through the metal loop, an optimized traditional matrix calculation method is proposed to obtain the accurate line fault current value. On this basis, with an actual engineering background, the equivalent circuit model of the four-terminal bipolar flexible DC power grid is established, and the influence of grounding position, grounding parameters, and current-limiting reactor on the fault current of the DC line is analyzed. Finally, simulation using the PSCAD software verifies the effectiveness and accuracy of the proposed method. The method proposed in this paper can provide the necessary bases and references for the selection of flexible DC grid equipment.
Highlights
The many advantages of voltage source converter-high-voltage, direct current (VSC-HVDC) make it suitable for new large-scale energy grid connection, grid interconnection (Allebrod et al, 2008; Perez et al, 2015; Guo et al, 2017; Li et al, 2021) and power quality control (Li et al, 2016), DC grid power flow control (Li et al, 2018), and other aspects
The topological structure of half-bridge converters is mostly used in engineering, and the DC circuit breaker is used to remove the fault before converter station blocking, so as to reduce the adverse impact caused by converter station blocking, control the capacity, and reduce the cost
The model parameters correlate to the Zhangbei DC project in China as follows: rated DC voltage is ±500 kV, rated DC current is 3 kA, rated power are 1,500/3,000/3,000/1,500 MW, sub-module capacitances are 8/15/15/11.2 mF, bridge arm inductances are 100/50/50/100 mH, the inductance of the DC bus smoothing reactor is 150 mH, and the inductance of the neutral smoothing reactor is 300 mH
Summary
The many advantages of voltage source converter-high-voltage, direct current (VSC-HVDC) make it suitable for new large-scale energy grid connection, grid interconnection (Allebrod et al, 2008; Perez et al, 2015; Guo et al, 2017; Li et al, 2021) and power quality control (Li et al, 2016), DC grid power flow control (Li et al, 2018), and other aspects. The following two pairs of equations are introduced here to further establish a DC equivalent model taking into account the AC effects: station; idcs represents the controlled DC current source of the equivalent circuit of the DC side of the MMC. The obtained equation contains a more intuitive physical meaning At this time, Ce = 6C0/N is defined as the equivalent capacitance of MMC; C0 represents the capacitance value of the converter sub-module; Pcon represents the injected power of the converter udcp udc/2 pcon ≈ pdc (18). The equivalent capacitance voltage matrix of the converter station of the flexible DC grid can be obtained by listing the KVL equations among the various discharge circuits:. The relationship between equivalent capacitance current and DC line current in the converter station is as follows:
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